A collapsed hole adds great expense to the drilling of a wellbore and can lead to the abandonment of the hole. Hole collapse can be caused by a number of drilling conditions including shale swelling, sloughing, and unconsolidated sands that cause a hole to wash out or collapse as soon as it is drilled. In these unstable formations, the bore hole can not be cased off and protected in time, when running a liner in a subsequent run after the hole was drilled.
Another cause of wellbore/hole collapse is an extreme pressure drop between adjoining formations. Drilling into a low pressure formation with a heavy mud that is designed to drill through an overlying high pressure zone will result in severe mud losses and simultaneous hole collapse. An opposite situation is encountered when a borehole is drilled through a first formation having a low formation pressure into a formation of substantially higher formation pressure, then there is the danger of fluids from the lower formation entering the borehole and damaging the upper formation. If the pressure difference is large enough, there is a risk of a blowout. If the mud weight is increased to prevent such a blowout, then the mud can damage the low pressure formation.
There is a need for an apparatus and method of drilling boreholes that avoids these problems. Such an invention should preferably reduce the operational time in its use. It should preferably be adaptable for use with directional drilling systems. It should reduce the exposure of the formations to the dynamic circulation pressure of the drilling mud and thereby reduce formation damage. A further desirable aspect is ro reduce the likelihood of getting stuck in the borehole. In addition, if the apparatus does get stuck, it should be possible to continue drilling ahead. The present invention satisfies this need.